The present invention relates to a flow completion system for producing oil or gas from a subsea well. More particularly, the invention relates to such a system which comprises a tubing hanger which is supported in a tubing spool that is positioned over a well bore and two distinct pressure-containing barriers which are associated with the tubing hanger and which isolate the well bore from the external environment.
A typical horizontal-type flow completion system, such as that disclosed in U.S. Pat. No. 6,039,119, comprises a wellhead housing which is installed at the upper end of a well bore, a tubing spool which is connected to the top of the wellhead housing and which includes a central bore that extends axially therethrough, an annular tubing hanger which is suspended in the central bore, and a tree cap which is installed in the central bore above the tubing hanger. The tubing hanger supports at least one tubing string that extends into the well bore and defines a tubing annulus surrounding the tubing string. In addition, the tubing hanger comprises a concentric production bore which communicates with the tubing string and a lateral production passageway that extends between the production bore and a production outlet in the tubing spool. The tubing spool also includes an annulus passageway which extends from the tubing annulus to an annulus outlet, and a workover passageway which extends from the annulus passageway to a portion of the central bore that is located above the tubing hanger. The annulus and workover passageways provide for communication between the tubing annulus and the portion of the central bore located above the tubing hanger during installation and workover of the flow completion system.
The regulations of certain countries pertaining to the subsea production of oil and gas require that the flow completion system provide at least two pressure-containing barriers between the well bore and the environment at all times. In the production mode of operation of the typical horizontal flow completion system, the first barrier is provided by a wireline plug that is installed in the production bore above the production passageway, in conjunction with an annular, usually metal seal which is positioned between the tubing hanger and the tubing spool above the production outlet. The second barrier is provided by the tree cap, which is sealed to the tubing spool by an annular, typically metal seal and which often includes an axial through bore which in turn is sealed by a wireline plug or other suitable closure member.
During installation of the flow completion system, the tubing spool is landed onto the wellhead housing, after which a blowout preventer (“BOP”) is installed onto the tubing spool by means of a riser deployed from a surface vessel. The tubing hanger is then lowered on a tubing hanger running tool (“THRT”) through the riser and the BOP and landed in the central bore of the tubing spool. After the THRT is retrieved, the tree cap is lowered on a dedicated tool through the riser and the BOP and landed in the central bore directly above the tubing hanger. After the tree cap is installed, the THRT is retrieved, the BOP is retrieved, and the flow completion system is ready to be put into production. During a typical workover of the flow completion system, the BOP and the riser are once again connected to the tubing spool, the tree cap is usually removed from the tubing spool, and the THRT is connected to the tubing hanger. Once the workover operations are completed, the THRT is retrieved and the tree cap is re-installed through the riser and the BOP. Then the THRT is retrieved, the BOP is retrieved, and the flow completion system is ready to be put back into production.
Since the tree cap is required to maintain well pressure in the event of a failure of the first barrier, the tree cap typically comprises a rigid metal body and a robust metal lockdown mechanism to firmly lock the body to the tubing spool. Consequently, the tree cap is usually too heavy to be installed by a remotely operated vehicle (“ROV”) and must instead be lowered from the surface vessel on a specially designed tree cap running tool. Thus, installation of the tree cap requires a special running trip, both during installation of the flow completion system and after a workover operation. Each such trip typically requires a significant amount of valuable rig time to complete, which necessarily increases the cost of completing and maintaining the well.
In addition, during retrieval of the THRT prior to installing the tree cap, debris within the riser often falls into the central bore of the tubing spool above the tubing hanger. Left unattended, this debris could foul the sealing surfaces of the central bore and thereby prevent the tree cap from forming an effective seal with the tubing spool. Therefore, before the tree cap is installed the central bore must be thoroughly cleaned, a process that consumes additional valuable rig time and increases the cost of completing and maintaining the well.